1. Field of the Invention
The present invention relates to a vapor phase epitaxial growth method of a compound semiconductor such as GaAs, InP and InGaAs and a system used therefor.
2. Description of the Prior Art
Recently, a demand for photoreceivers made of III-V compound semiconductors such as avalanche photodiodes (APDs) and PIN photodiodes (PIN-PDs) in long-wavelength optical communication systems has increased rapidly.
The APDs and PIN-PDs are generally formed in heteroepitaxial layers which are formed on a n.sup.+ -InP substrate. The layers are generally obtained by a vapor phase epitaxy (VPE) using hydrides or metal organic compounds (See Y. SUGIMOTO ET AL., "ELECTRONICS LETTERS", Vol 20, No. 16, PP 653-654, Aug. 2, 1984).
FIG. 1 shows an example of a conventional crystal growth system used for hydride VPE of a III-V compound semiconductor.
In FIG. 1, there is provided with a reactor 52 in a crystal growing furnace 51. In the reactor 52, an upper reaction chamber 59 and a lower reaction chamber 60 are formed, and a substrate holder 57 for holding a semiconductor substrate 58 is provided in the downstream side (the right side in FIG. 1) in the reactor 52. The holder 58 can be rotated around a horizontal shaft (not shown) and set at a position in the upper reaction chamber 59, which is shown by solid lines in FIG. 1, and another position in the lower reaction chamber 60, which is shown by broken lines in FIG. 1, alternately. The holder 57 is located in the low-temperature area (approximately 700.degree. C.) in the reactor 52.
In the reaction chambers 59 and 60, there are gas diffusers 56a and 56b for diffusing gases supplied in the chambers 59 and 60, respectively. The respective diffusers 56a and 56b are composed of several plates having many holes.
Materials 55a and 55b stored in containers for the group III component of the III-V semiconductor are disposed in the upstream side of the chamber 59 and materials 55c and 55d stored in containers for the group III component thereof are disposed in the upstream side of the chamber 60. The materials 55a, 55b, 55c and 55d are located in the high-temperature area (approximately 850.degree. C.) in the reactor 52.
A carrier gas for the materials 55a and 55b is supplied to the chamber 59 through tubes 53a and 53b which are connected to the upstream end of the chamber 59, and a carrier gas for the materials 55c and 55d is supplied to the chamber 60 through tubes 53c and 53d which are connected to the upstream end of the chamber 60.
Gaseous materials for the group V component of the III-V semiconductor (and a gaseous n- or p- dopant, when required) is supplied through tubes 54a and 54b to the chamber 59 and 60, respectively. Ends of the tube 54a and 54b are located near the downstream ends of the chambers 59 and 60 respectively so that the gaseous materials may not chemically react to the materials 55a, 55b, 55c and 55d.
The VPE of the III-V compound semiconductor using the above-identified system is performed as follows:
First, the carrier gas (e.g., HCl) for the materials 55a and 55b (e.g., Ga or In) is introduced through the tube 53a and 53b into the high-temperature area of the chamber 59 heated in the furnace 51. The carrier gas chemically react to the materials 55a and 55b to generate a gas (e.g., GaCl or InCl), which is used for the group III component material. On the other hand, a gaseous hydride (e.g., AsH.sub.3 or PH.sub.3) of the group V component is introduced through the tube 54a into the high-temperature area of the chamber 59.
The gaseous group III component material and the gaseous hydride of the group V component in the high-temperature area of the chamber 59 are mixed with each other near the end of the tube 54a and diffused by the diffuser 56 to flow into the low-temperature area thereof, in which the substrate holder 57 is provided. The gaseous group III and V materials in the low-temperature area chemically react to each other and thereby a first III-V semiconductor epitaxial layer having a predetermined composition grows on the substrate 58.
Next, after the substrate holder 57 is rotated in the reactor 52 and set at the position in the lower reaction chamber 60, a carrier gas for the group III component is introduced through the tube 53c and 53d into the chamber 60 and at the same time, a gaseous hydride (e.g., PH.sub.3) of the group V component is introduced through the tube 54b into the chamber 60. Then, a second III-V semiconductor epitaxial layer having a predetermined composition grows on the first epitaxial layer.
Concretely, in case that InGaAs/InP heteroepitaxial layers are formed on an n.sup.+ -InP substrate 58, metallic gallium (Ga) and metallic indium (In) are used for group III component materials, and hydrogen chloride (HCl) is used for a carrier gas for the group III component materials and arsine (AsH.sub.3) is used for a group V component material in the upper reaction chamber 59. Besides, metallic Indium (In) is used for a group III component material and phosphine (PH.sub.3) is used for a group V component material in the lower reaction chamber 60.
First, the substrate holder 57 is set at the position in the lower reaction chamber 60 and then an indium phosphide (InP) layer grows on the substrate 58. Next, the holder 57 is moved and set at the position in the upper reaction chamber 59, and then an indium gallium arsenide (InGaAs) layer grows on the InP layer. Thus, InGaAs/InP heteroepitaxial layers are formed on the n.sup.+ -InP substrate 58.
If an n-dopant gas such as silicon (Si) or sulfur (S) is introduced into the chamber 59 or 60 during the epitaxial growth, an n-epitaxial layer grows.
With the above-identified VPE, supply of HCl as the carrier gas and the PH.sub.3 as the group V component material are stopped when the growth of the InP layer in the chamber 60 has finished, and at the same time, the holder 57 is moved and set at the position in the chamber 59. The gaseous HCl and PH.sub.3 remain in the chamber 60 when the supply thereof is stopped, and the rotation speed of the holder 57 is set as low as approximately 10 rpm with regard to falling of the substrate 58. Therefore, the InP layer epitaxially grows slightly in a short time from the stopping of the supply of the HCl and PH.sub.3 in the chamber 60 to start of the growth of the InGaAs layer in the chamber 59. In addition, the InGaAs and InP layers grow in the HCl and PH.sub.3 which are not in stationary states.
Accordingly, a transition region of crystal (semiconductor) composition in which the composition is not constant is generated at a boundary between the InP and InGaAs layers and as a result, there arises a problem that heteroepitaxial layers having a layer structure as designed cannot be obtained. The transition region adversely affects device characteristics.
In addition, in case that the heteroepitaxial layers contain an n- or p-layer with impurity doped, a transition region of carrier (dopant) concentration in which the concentration thereof changes loosely is generated at the boundary between the InP and InGaAs layers due to a remaining dopant gas. As a result, there arises a problem that a carrier (dopant) profile in which the carrier concentration changes sharply at the boundary of adjacent layers cannot be obtained.
FIG. 2 shows an example of InGaAs/InP heteroepitaxial layers on a n.sup.+ -InP substrate obtained by the above-identified VPE. In the layers, it was found that widths of transition regions of crystal composition at boundaries between an i-InGaAs layer (thickness: 1 .mu.m) and adjacent n-InP layers (thickness: 1 .mu.m, carrier concentration: 2.times.10.sup.16 cm.sup.-3) are approximately 20 nm by the transmission electron microscopy (TEM), and that widths of transition regions of carrier concentration at the boundaries are approximately 30 nm by the secondary ion mass spectroscopy (SIMS).